1. Field of the Invention
This invention relates to brushless excitation systems for large electrical generators. The ac exciter of the excitation system has a diode wheel associated with it which operates to rectify ac excitation into dc current for supply to the generator rotor field winding. The invention provides a saturable reactor diode snubber assembly composed of multiple saturable reactor film members which are disposed along the length of coils in the rotating armature of the main exciter which film members result in reduction of diode stored charge and reverse recovery energy.
2. Background Information
Large electrical generators require external excitation which is provided by the exciter. Conventional systems provide exciter output connected to the generator field through a series of slip rings and brushes. An alternative system is a brushless exciter which is joined directly to the generator field winding without the need for U brushes and slip rings. The present invention relates to the brushless excitation system which employs a stationary field and generates an alternating current using a rotating armature. The ac must be rectified to supply dc to the generator field. This brushless system utilizes a rotating diode wheel rectifier. The rotating rectifier has the advantage that excitation is assured as long as the turbine generator shaft is rotating as the brushless exciter derives its power from the main generator shaft. This enables the direct-coupled, shaft-driven exciter to be an extremely reliable machine.
Ac output from the exciter is rectified by wheel-mounted diodes on the rotor shaft producing a dc voltage. The excitation voltage is fed directly to the field winding in the generator rotor by short leads located in the center of the shaft or which pass to diametrically opposed slots on the periphery of the exciter to generator coupling.
One difficulty faced with solid state rectification systems used with brushless exciters is that power semiconductor turnoff transients are generated. Diode turnoff in brushless exciters results in voltage spikes which can harm the system and can ultimately lead to unnecessary system stoppage. One approach to lowering the level of voltage spikes at diode turnoff involves placing a capacitor across the diode which absorbs the stored charge on reverse recovery and thereby eliminates the problems due to voltage spikes caused during reverse recovery.
Such capacitors, however, require space and add further weight to assemblies. Space and weight restrictions on the diode wheels are severe due to the 1800 rpm or 3600 rpm operating condition. In order to withstand the centrifugal forces, the diode snubber capacitor designs result in bulky assemblies which occupy substantial space which would preferably be utilized for additional diode modules, heat sinks, and air space for cooling purposes.
In other applications, such as ac phase conductors, it has been known to employ saturable reactors to mitigate power semiconductor turn-on/turnoff transients. However, the problem in the case of a brushless exciter is that there has been no obvious place to install a saturable reactor without jeopardizing the mechanical reliability of the system.
There remains a need, therefore, for an improved diode snubber assembly which acts to eliminate or reduce the effects of voltage spikes and stored charge transients which occur during diode turnoff.
There remains a further need for such a diode snubber which can achieve higher power densities with a more spacious layout allowing increased range of design opportunities for diode modules and heat sinks, and which can also be fitted to existing systems without any rewinding of the exciter.